The present invention relates to a classifier and its controller, the classifier being operable in a vertical mill, for example, to guide a powdery material by means of a gas flow, and to selectively draw off a portion of the powdery material according to the particle size of the powdery material.
FIG. 26 is a simplified sectional view showing a prior art static type of vertical mill 1. With reference to FIG. 26, in the casing 1a of the vertical mill 1 is mounted a table 2 having a vertical rotating axis, and the table 2 is rotated by a drive 3. This table 2 includes a table liner 2a for crushing powdery materials. Above the table liner 2a, a plurality of angularly spaced crushing rollers 4 are arranged around the circumference of the table. Each crushing roller 4 is rotatably connected to an arm 5 which swings on a pivotal axis 6 so that the angle between the table 2 and the arm 5 can be varied. An upper end of the arm is connected to a pressurizing device 7 which extends out of the casing 1a. This pressurizing means 7 presses on the arm 5 in an elastic manner thereby pressing the crushing roller 4 against the table liner 2a.
Above the table 2 is installed a feed tube 8 which feeds a raw material, such as a granular material, into the casing and onto the table. Further, above the table 2 is installed a classifier 9 which consists of a generally funnel-like cone 10 and classifying blades 11. In the top plate 12 of the casing 1a of the vertical mill 1, an outlet port 13 is provided for drawing the powdery material out of the casing 1a. In the casing 1a and beneath the table 2 are provided blast or intake ports 14 for supplying a gas flow around the table to raise the powdery material upwardly through the casing 1a, as will be explained later.
In a vertical mill 1 of the above-mentioned configuration, a powdery material fed through the feed tube 8 drops on the table 2. As the table 2 is rotated by the driving means 3, the powdery material is moved by the centrifugal action into a gap between the table liner 2a and the crushing rollers 4. The powdery material thus crushed between the table 2a and the crushing rollers 4 is caused to rise in the casing 1a by the gas being fed through the blast ports 14. The powdery material moves up around the outside of the cone 10 and enters, through a guide passage 15 between the cone 10 and the top plate 12, into the classifier through the blades 11. Upon entering, a portion of the powdery material, wherein the particle size is equal to or greater than a predetermined value, is driven downwardly by the classifying blades through the interior of the cone 10, and is guided by the cone 10 and drops again on the table 2a. The portion of the powdery material of which the particle size is smaller than the predetermined value is lifted out of the casing 1a through the outlet port 13 by the gas flow from the blast port 14. The powdery material which drops through the cone 10 down to the table 2 is mixed with the powdery material being fed by the feed tube 8 and it is again crushed between the table liner 2a and the crushing rollers 4.
The vertical mill 1 which crushes material in the aforementioned manner is simple in construction, but it is not capable of producing, at the outlet port 13, a powdery material with an easily or freely-selected particle size distribution. In other words, the powdery material obtainable at the outlet port 13 can be adjusted in fineness (cm.sup.2 /g) so that it is not larger than a predetermined value by adjusting the angle of the classifying blades 11, but it is not possible to discharge a powdery material having a freely-selected particle size distribution.
FIG. 27 shows a simplified sectional view of another prior art rotary blade type of vertical mill 20, and FIG. 28 is a graph for explaining the classifying function of the vertical mill 20. This prior art mill is generally similar to the prior art mill shown in FIG. 26, and the corresponding parts are indicated by the same reference numbers. The present prior art mill is characterized in that a plurality of circumferentially spaced rotary blades 21 are provided in the upper portion of the casing 1a, in place of the cone 10 and the classifying blades 11 which constitute the classifier 9 of the prior art mill shown in FIG. 26.
The rotary blades 21 are secured, at their lower ends as shown in FIG. 27, to a support member 22, and the support member 22, in turn, is fixed to a rotary shaft 24 which is rotatably driven by a drive 23.
In the vertical mill 20 of the above-mentioned configuration, a powdery material fed into the mill by the feed tube 8 rises, after passing through the processes similar to those described in connection with FIG. 26, in the casing 1a. In rising, the powdery material moves in such a manner as to pass, together with the gas from the blast port 14, through the spaces between the plurality of rotating blades 21. Since the blades 21 are being driven to rotate as explained above, a portion of the powdery material, the particle size of which is greater a certain predetermined value, is given a large centrifugal force and forced to drop downwardly in the casing 1a. On the other hand, the portion of the powdery material the particle size of which is equal to or smaller than the predetermined value, passes through the spaces between the rotary blades 21 and moves out of the casing 1a through the outlet port 13. The portion of the powdery material having the excessive particle size, which drops downwardly in the casing 1a, is crushed again on the table 2.
The vertical mill 20 of the above-mentioned configuration is capable of adjusting the particle size of the powdery material leaving the outlet port 13 by altering the rotational speed of the rotary blades 21. With reference to FIG. 27, when the rotary blades 21 are rotating at a constant speed, the particle size distribution obtained at the outlet port 13 is indicated by the line 100 of FIG. 28.
When the rotational speed of the rotary blades 21 is reduced, the configuration of the particle sizes of the powdery material leaving the outlet port 13 will be as shown by the line 101 of FIG. 28, according to the Rosin-Rammler paper. If the angle between the line 100 and the axis of the abscissa, and the angle between the line 101 and the axis of the abscissa are denoted by .theta.1 and .theta.2 respectively, the tangential values N obtained from .theta.1 and .theta.2 are expressed by the following equations: EQU N1=tan .theta.1 (1) EQU N2=tan .theta.2 (2)
As shown in FIG. 28, the values N representing the configuration of particle diameters of the powdery material satisfy the following relation: EQU N1.apprxeq.N2 (3)
Although a predetermined fineness (cm.sup.2 /g) can be freely selected by changing the rotational speed of the rotary blades 21, it is not possible to obtain a freely selected configuration of particle sizes of the powdery material. In FIG. 28, the fineness of the powdery material of the line 100 is higher than that of the line 101 since the overall particle sizes of the line 100 are smaller than those of the line 101. However, it is not possible to adjust N.sub.1 and N.sub.2 of these lines. In the case of FIG. 26, the angular adjustment of the classifying blades 11 corresponds in results obtained to the rotational speed adjustment of the rotary blades 21 shown in FIG. 27.
In the vertical mill 20 shown in FIG. 27, when clinkers, for example, are to be crushed, it is desirable, in view of the strength achieved when water is added to the cement, and the attendant cost, to set the value of N, and accordingly the configuration of the particle sizes of the powdery material as shown in FIG. 28, so that the powdery material consists of a considerably wide range of particle sizes. In the vertical mill 20, however, since the crushing time of the powdery material is short, there is a problem in that the portion of the powdery material which circulates in the casing 1a becomes larger, and in turn the value N gets larger, or the configuration of the particle sizes of the powdery material obtainable at the outlet port 13 is extremely narrow.
FIG. 29 is a simplified sectional view of a prior art static-rotary blade type vertical mill 30. Corresponding parts of the vertical mill 30, which are similar to the above-mentioned prior art mills, are denoted by the same reference numbers. In this prior art vertical mill 30, a classifier 9 consisting of a cone 10 and classifying blades 11 such as those shown in FIG. 26, and the rotary blades 21 such as those shown in FIG. 27, are installed in combination. The raw material, fed through the feed port 8, rises as powdery material in the casing 1a, through the processes explained in connection with the above-mentioned prior art mills. The powdery material thus raised is guided into the cone 10 through a guide passage 15 and between the classifying blades 11. Upon entry, a portion of the powdery material of which the particle sizes are equal to or greater than a predetermined particle size, is dropped by the classifying blades 11 along the inner wall of the cone 10 to be collected on the table 2. The portion of the powdery material which has not been so collected is classified, as explained above, by the rotary blades 21 which are driven by the drive 23, and the powdery material thus classified is taken out of the casing 1a through the outlet port 13. The remaining portion of the powdery material drops through the cone 10 onto the table 2 and is again crushed.
The vertical mill 30 of such a configuration also has a problem similar to that pointed out for the vertical mill 20 of FIG. 27. Namely, the distribution or configuration of the particle sizes of the powdery material obtainable at the outlet port 13 is narrow, and changing the speed of rotation of the rotary blades 21 changes the central or average value of the particle size distribution of the powdery material, but not the range of particle size distribution of the powdery material; a freely selected range of distribution cannot be obtained.
Thus the problem common to the prior art mills is that it is difficult to adjust the range of configuration of the particle sizes of the powdery material obtainable from the outlet port 13 at any desired level to suit the intended use of the powdery material.
Therefore it is a primary objective of the present invention to provide a classifier and a controller therefor, the classifier being capable of solving the above-mentioned problem and of freely setting the range of the particle size distribution of the powdery material from the classifier at a predetermined desired value.